Conventionally, a projection type display (projector) for displaying an image on a screen by projecting light has been known. As a light source for this projector, a discharge lamp such as a metal halide lamp has been used, and a discharge lamp lighting device for starting up the discharge lamp and for maintaining the lighting has been utilized. A conventional discharge lamp lighting device is disclosed in Patent Document 1, for example. FIG. 21 is a circuit diagram showing a configuration of a conventional discharge lamp lighting device. In FIG. 21, 100 denotes the conventional discharge lamp lighting device, which includes: a line filter 102 connected to an alternating current power supply 101; a rectifier bridge 103; a step-up converter 104; a step-down converter 105; and an igniter 106 that is operated when a lamp 107 is started up. The step-up converter 104 converts commercial AC power to a direct current, and may thus be called a “power supply” in combination with the line filter 102 and the rectifier bridge 103. On the other hand, the step-down converter 105 chops and controls current or voltage when the converter is configured as a switching regulator type, and may thus be called a “step-down chopper”. Similarly, the step-up converter may be called a “step-up chopper”. The rectifier bridge 103 includes four diodes, and may be called a “diode bridge”.
Furthermore, the step-down converter 105 carries out control so that the product of the voltage of both ends of the lamp 107 and the current flowing therethrough is kept constant, and may thus be called a “ballast”. The power to be supplied to the lamp 107 has a rated value, e.g., 200 W±10%, while the voltage of both ends of the lamp 107 varies due to the life span or the like thereof, and therefore, the power to be supplied is kept within a certain range by controlling the current amount. Further, the lamp 107 may be an alternating current type lamp in which electric potentials of electrodes at two spots, in general, alternate periodically, and a direct current type lamp in which the polarity of electric potential is always constant. In FIG. 21, the device is configured to light a direct current type lamp. For example, an inverter including four FETs is provided between the step-down converter 105 and the igniter 106 to convert the direct current output from the step-down converter 105 to alternating current, thus making it possible to provide for an alternating current lamp.
An alternating current of 100 V to 240 V outputted from the alternating current power supply 101 is rectified in the rectifier bridge 103 via the line filter 102, and a peak voltage (144 V to 340 V), which is approximately √2 times the alternating current, is generated at an output of the rectifier bridge 103. Then, the peak voltage is increased to a direct current of 370 V to 400 V by the step-up converter 104. The step-down converter 105, serving as a ballast, reduces the voltage so that it becomes 100 V to 200 V at the time of glow discharge of the lamp 107, 15 V at the time of initial arc discharge, and 60 V at the time of steady arc discharge. Further, the igniter 106 is a circuit used at the time of the breakdown of the lamp 107, and the igniter 106 is operated when an open-circuit voltage, which is an output of the step-down converter 105, is in the range of 200 V to 300 V. The igniter 106 generates a voltage of about several kV to about several tens of kV, and causes breakdown in the discharge lamp internal gas, thereby starting up the lamp.
Furthermore, a high-intensity discharge lamp lighting device for lighting an alternating current-driven lamp is also known (for example, Patent Document 2). FIG. 22 shows a conventional high-intensity discharge lamp lighting device for lighting an alternating current-driven lamp. In this diagram, 110 denotes the conventional high-intensity discharge lamp lighting device, which includes: a line filter 112 connected to an alternating current power supply 111; a rectifier bridge 113; a step-up transformer 114; a resonant switching power supply control circuit IC; a voltage-controlling oscillator VCO; and a current detection resistor RD for detecting a current flowing through the lamp 117.
The step-up transformer 114 includes a primary coil NP, a secondary coil NS and a feedback coil Nf for a voltage-resonant inverter, and a power switch element Q1 and the rectifier bridge 113 are connected in series to this primary coil NP. A resistor R2 is a startup resistor. When the high-intensity discharge lamp lighting device 110 is started up, and the output voltage of the rectifier bridge 113 is increased from zero volt, power is supplied to the resonant switching power supply control circuit IC1 via the startup resistor R2, thus operating this circuit. In addition to the voltage-controlling oscillator VCO, the resonant switching power supply control circuit IC1 includes a one-shot multivibrator, an error amplifier, a pulse frequency modulator and the like, which are not shown.
The lamp 117, a choke coil L1 and the current detection resistor RD are connected in series to the secondary coil NS of the step-up transformer 114. The conventional high-intensity discharge lamp lighting device 110 carries out constant current control of the current flowing through the lamp 117. Specifically, the lamp current is detected by the current detection resistor RD, and the rectified and smoothed direct current is connected to a control input terminal of the resonant switching power supply control circuit IC1, thereby carrying out the constant current control. When the lamp current has been increased for some reason, the voltage of both ends of the detection resistor RD is increased, and the output voltage of the unshown error amplifier of the resonant switching power supply control circuit IC1 is increased; as a result, the oscillating frequency of the voltage-controlling oscillator VCO is increased to reduce the lamp current.    [Patent Document 1] Japanese Patent Application Laid-Open No. 9-320772    [Patent Document 2] Japanese Patent Application Laid-Open No. 7-302688